The present invention relates to energy harvesting mechanisms for generating electrical power. More particularly, the present invention relates to a thermo-electrochemical device that converts heat energy from an environment in which the device is located into electrical power.
It has long been a goal to develop an engine that operates on thermal energy that is freely available in the ambient environment. Consistent with the second law of thermodynamics, prior attempts at such thermal-energy-harvesting devices required two distinct sources of thermal energy, namely, a heat source and a heat sink for supplying and removing heat, respectively, at different temperatures simultaneously. A heat-source and heat-sink pair having two distinct, spaced-apart temperatures typically does not occur naturally and/or plentifully, and thus is generally difficult to access. Therefore, because ambient heat at a single atmospheric temperature is more abundant and available than a simultaneous dual-heat source, a device for harnessing single-source ambient heat is more desirable than a device that requires a dual-heat source.
In U.S. Pat. No. 6,899,967, the present inventor disclosed a device that relies on cyclic temperature changes in the environment to produce the needed simultaneous dual-heat source. The needed temperature difference was artificially created by providing a mass of material that has significant heat capacity. The prior art device is a thermo-electrochemical converter that operates on a pressure difference between two metal-hydride chambers separated by a membrane electrode assembly (MEA). In the prior art invention, one metal-hydride chamber is exposed to the ambient environment, while the other is insulated and thermally stabilized. A thermal mass is coupled to the stabilized chamber to act as a heat source/sink material. Insulation may be used to thermally isolate the thermal-mass material from the environment in order to enhance the temperature difference produced. It absorbs heat and stores it during periods of elevated ambient temperature and releases that heat to function as an elevated-temperature heat source during periods of reduced ambient temperatures. As such, changes in the temperature of the insulated metal hydride coupled to the thermal mass will always lag temperature changes in its environment. Thus, a converter coupled between the thermal mass and the environment will be subjected to a simultaneous temperature differential needed for the device to operate.
A major limitation encountered with the prior art invention is associated with the need to have a device that is capable of scavenging power in a relatively efficient manner. A major limitation in achieving efficient operation is associated with the difficulty of creating a significant temperature difference between components. This is particularly true for a small device. The close proximity of the components in a small device allows for large parasitic heat conduction losses between the high and low temperature sections, which results in heat transfer between the two sections without performing power generation. This problem becomes particularly challenging when the rate of change in the environmental temperature is slow.
To eliminate the need for artificially creating a temperature differential, the present inventor disclosed in U.S. Pat. No. 9,266,085 an ambient heat engine (AHE) employing a heat source and heat sink that operates on the Sterling thermodynamic cycle and is consistent with the second law of thermodynamics. However, for the AHE, the heat source and heat sink do not occur simultaneously.
Ideally, the entire engine maintains a state of thermal equilibrium with its environment. It overcomes the limitation of past thermoelectric conversion systems by eliminating the need to maintain or create a simultaneous temperature differential across the device. The engine incorporates an electrochemical system for converting heat from its environment into electrical energy. The AHE operating principal is based on the thermo-galvanic effect wherein the voltage of an electrochemical cell is a direct function of its temperature and state of charge. The engine is driven by changes in temperature transients which occur in its ambient environment. The cell converts ambient heat into electricity by being charged at one temperature and voltage and then discharged at a different temperature and higher voltage.
A preferred embodiment is disclosed that employs hydrogen concentration electrochemical cells wherein voltage potentials are created using metal hydrides to apply hydrogen pressure differentials across proton conductive membrane electrode assemblies. However, the invention is equally applicable to other electrochemical cells having appreciable thermo-galvanic properties. A Copper Hexacyanoferrate (CuHCF) electrochemical cell is one alternate example. Gang Chen, Department of Mechanical Engineering at Massachusetts Institute of Technology, and Yi Cui, Department of Materials Science and Engineering at Stanford University & Stanford Institute for Materials and Energy Sciences SLAC National Accelerator Laboratory, reported use of a Copper Hexacyanoferrate (CuHCF) electrochemical cell to convert waste heat from industrial and other processes into electricity in NATURE COMMUNICATIONS|5:3942|DOI: 10.1038/ncomms4942|www.nature.com/nature communications.
The voltage of a thermo-galvanic electrochemical cell is a direct function of its temperature and state of charge. Thermo-galvanic cells convert heat into electricity by being charged at one temperature and voltage and then discharged at a different temperature and higher voltage. Use of thermos-galvanic cells for ambient energy harvesting is most attractive for applications where long term operation is needed and where periodic manual refueling of a power generator or replacement or recharge of batteries is not practical. Unfortunately, galvanic cells are subject to internal current leakage, molecular diffusion and other self-discharge mechanisms. Thus, it can be appreciated that a need exists for a device that produces electrical power using heat that is freely available from its ambient environment, and that overcomes the disadvantages and shortcomings of previous thermal converters that are subject to long term self-discharge and loss of effective operation.